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Stereoselectivity glycosyl radicals

Stereoselective addition of ribofuranosyl radicals to a, 9-unsaturated carbonyl compounds was exploited by Araki [22] in the formal synthesis of the antiobiotic Showdomycin 16 (Scheme 4). Barton [23] provided a more direct synthesis of 16 by addition of the ribofuranosyl radical, generated using the sugar telluride 17, to maleimide. Ribofuranosyl radicals also undergo addition to Tt-systems of electron-deficient aromatics such as 18, which provides a direct route to C-nucleosides [24], Addition of glycosyl radical donors to allyltributyltin was introduced by Keck... [Pg.1039]

Crich et al. have used the radical translocation method for the generation of anomeric glycosyl radicals and has also applied this to the stereoselective synthesis of P-mannoside (Scheme 22). Photolysis of the readily available a-mannoside 70, which possesses a C2-bromoethoxy group, with tributyltin hydride in the presence of a catalytic amount of AIBN affords P-mannoside 71 as a major product after acid hydrolysis of the resulting C2-acetal group. The reaction proceeds through the formation of radical 72... [Pg.72]

Anomeric halides follow the typical reactivity order F < Cl < Br < I for nucleophilic substitutions. They have been used in stereoselective O-glycosylation, nucleophilic displacement, and carbanion as well as in radical reactions. [Pg.80]

Radical reduction of 1-nitro-C-glycosyl compounds. In 1983, Vasella and co-workers125 reported a stereoselective method for the synthesis of a-C-mannopyranosyl compounds by reduction of 1-nitro-C-mannopyranosyl derivatives with Bu3SnH in the presence of a,a -azoisobutyronitrile (AIBN) radical initiator. These reactions involve the formation of anomeric centered radicals. Thus, in the case of d-manno configuration as in 140, the 1,2-cts-C-pyranosyl compound 145 was obtained in 84% yield. The intermediate pyranosyl radicals 143 prefer a-attack by the tin hydride. Thus for D-glucopyranosyl derivatives, the corresponding l,2-tra x-C-pyranosyl compound 144 is obtained preferentially (Scheme 47). [Pg.57]

Stereoelectronic effects can be invoked for the radical reaction at anomeric centre of carbohydrates. The high stereoselective preparation of a-substituted C-glycosyl phosphonates in a a p ratio of 98 2 was achieved by reductive addition of bromide 2 to a-phosphonoacrylate (Reaction 7.5) [10]. Yields (in parentheses) depend on the sugar configuration D-galacto (80%), D-manno (47 %), D-gluco (30 %) and L-fuco (62 %). [Pg.146]

Carbohydrates with a hydrazono ester at the C-2 position underwent intramolecular radical C-glycosylation upon reaction with tin hydride and AIBN, to afford the corresponding lactone in good yield (O Scheme 47). Although the stereoselectivities of the amino acid moiety were generally low, the methods have been used for the synthesis of a variety of C-glycosyl amino acids [73], The following reaction was a key step in the total synthesis of furanomycin, an antibiotic. [Pg.790]


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See also in sourсe #XX -- [ Pg.2 , Pg.3 , Pg.4 , Pg.5 , Pg.6 , Pg.7 , Pg.8 , Pg.9 , Pg.12 ]




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